1. Put oven on GM 4 / 180.
2. Mix butter and flour to the texture of fine breadcrumbs (Kenwood K Beater does it best).
3. Add Sugar, Choc Chips and Vanilla Essence and mix in gently.
...and now for the tricky bit...
4. Beat in the egg slowly paying close attention to the texture of the mixture. It will turn from powdery to a solid lump fairly quickly, and BEFORE it does this you need to decide if you will need to add some milk or not. If you have followed the measurements exactly you should not really need to. The finished mixture should be the texture of play dough... not sticky and not crumbly.
5. Break in to 8 pieces and place in muffin cases on a baking tray.
6. Bake on GM4 / 180 for 15-20 Minutes until golden brown and firmish.
7. Allow to stand to cool for at least an hour before eating.